Leaf-base (fig. 22).—The leaf-base is elongated and sheathing, colourless, and thickened considerably near the centre, but thin and membranous Fig. 22.—Plantago triandra. Transverse section of petiole (× 48). a, aqueous tissue; b, xylem; c, phloem; d, bundle-sheath. at the margins. The upper epidermis consists of large, thin-walled, colourless cells, without any stomata. The outer walls are slightly thicker than the others. The lower epidermis is composed of smaller cells than the upper, and the walls are somewhat thicker. The ground-tissue consists of large, rounded, thin-walled cells in which the protoplasm is reduced to a very thin layer lining the walls; this tissue is for water-storage. The vascular bundles are small, and of the same structure as in the leaf. Fig. 23.—Plantago triandra. Transverse section of root (× 175). a, suberized cells; b, cortex; c, endodermis; d, pericycle; e, phloem; f, xylem. Root (fig. 23).—The vascular cylinder is small, consisting of a central mass of xylem, composed of elements of small diameter, and with thickened, pitted walls; surrounding the xylem is a narrow band of phloem. The pericycle is a single layer of thin-walled cells. The endodermis is well marked, and consists of cells somewhat larger than the cells of the pericycle and with thicker, slightly suberized walls.